An Heir is the next in line to become king.
When a king dies with no wife, or children through mistakes or by other means, dynasties can change.. It is not generally considered proper for a monarch to go traipsing about the world by himself, questing in search of women and adventure. His duties are at home, near to his subjects. This is especially true if the king has no heirs. It is usually the job of the Prime Minister to set up banquets and parities to help the king procure a wife and thus an heir can be born.
In Daventry the heir during succession is usually the first born son (or daughter) of the reigning king. But is apparently traditionally and almost always male, and often related to the king, usually the eldest prince. Princesses must marry, and their husbands become king. The Heir during succession of rule is usually the first born son (or daughter) of reigning king. However, when there is no direct heir, it is maybe passed onto the wife, or other close relatives of the king or nobility. King Edward, for example had no son to inherit the throne, nor a daughter to gladden his heart.
When there is no close or direct relatives as was the case with the previous King Edward, then the king chooses a suitable heir from noble families (knights or others). These maybe distantly related or not. Though the heir need not necessarily be of nobility but someone trusted by the king. In Edward's case he chose Graham, his favorite knight, and the son of another knight, his best friend Hereward.
The young knight had been chosen in part based on a vision the King had witnessed in kingdom's lost magic mirror some decades before in which showed the young knight as the next king (he had believed it had been his 'future' son at the time). When Edward had no son, and after his wife died, Edward adopted the knight as his son and heir.
As was the case with Edward, other relatives even those distant, or another respected noble or knight in the kingdom may be chosen to become the heir. Though they may be challenged to prove they are worthy before hand. Graham for example was sent on the quest to find the three lost treasures of Daventry. Another indication that Graham was adopted as 'heir', is that he calls graham, his 'son'. But beyond that he may also be distantly related by blood as well.
In the world of King's Quest 'to heir is human', and so is to 'error'.
Alexander and Rosella later became the 'heirs' to the throne of Daventry. Though with Alexander having gone missing for eighteen years (though he regained the honor of being heir upon his return) and, later becoming King of the Green Isles (and his line being heirs to that throne), this was honor was passed onto Rosella.
Alexander at least still held onto the family heirloom insignia ring granted to him as Prince of Daventry, which was later to become Cassima's wedding ring. But its unclear if it continued to be a sign of his inheritance of Daventry at that point. But Cassima is at least considered part of the Royal Family of Daventry herself.
Behind the scenesEdit
Alexander was intended to be the original first 'heir' to the kingdom over Daventry, placing his position over Rosella, as noted in the subtitles of his games. He ended up leaving Daventry for good and ended up ruling another land.
The title of his second game "Heir Today, Gone Tomorrow", even foreshadows that he was to leave Daventry, and pass on inheritance to become the King of the Green Isles. As he went missing as a child, the honor was passed to Rosella (she was always intended to be next in line in the original universe). But based on KQ3 and KQ6's subtitles (To Heir is Human) he regained his inheritance on his return, but apparently gave it up when he decided to leave Daventry for good and rule another land, leaving Rosella in his place.
Though the question of if he could rule two kingdoms was technically never answered as the original series ended with KQ8. There is only forshadowing that Rosella would become Queen of Daventry in expanded universe works.
It's at least noted at the end of KQ3 and beginning of KQ4 that the hat tossing event appears to indicate both Graham acknowleding his twins as the heirs to the kingdom, but possibly deciding who would become his next in line and would take on the mantel of adventurer or Questor. Though he may have intended for both to share the duties of joint rule over the kingdom (much as the Pevensies of Narnia). But that potential future never manifested and Graham's heart attack changed the subject to more dire matters.